The present invention relates to a process for producing polycaproamide (polyamide 6) and copolymers of caprolactam having a low content of monomers and oligomers, and which are obtained by separating the low molecular weight reaction products and the .epsilon.-caprolactam which has not been involved in the reaction from melts of the said polymers after the synthesis. The process is useful in the production of polyamide 6 (PA6) and copolymers based thereon.
The polycaproamide produced by polymerization of .epsilon.-caprolactam always contains a certain amount of low molecular weight compounds (mixture of low molecular weight compounds comprising .epsilon.-caprolactam and oligomers), which amount is determined by the synthesis temperature and is a consequence of the equilibrium state of the polymer-monomer system at a given temperature.
In the case of hydrolytic or anionic polymerization at a temperature of 240.degree. to 280.degree. C the proportion of low molecular weight compounds is 7 to 14%.
The low molecular weight compounds contained in the polymer reduce the physical-mechanical properties of the products and cause considerable difficulties during processing.
It has been proved on the basis of practical work with PA6, that the content of low molecular weight compounds should not exceed 1.5% in technical injection molding grades and should not exceed 1 to 2% in grades intended for fibers. Such properties in polyamide 6 are only achieved at the present time in the course of heat-and-water extraction of the low molecular weight compounds from the finished polymer granules. This procedure is carried out by a discontinuous or continuous process at a temperature of 100.degree. to 200.degree. C and a residence time of 5 to 20 hours. The consumption of demineralized water is on average 2 t/t of polymer.
After the heat-and-water extraction, the rinsing water, which contains the low molecular weight compounds, is subjected to concentration by evaporation to 50% of the solution with subsequent regeneration of caprolactam. The polymer requires prolonged drying. Overall, the process is characterized by high energy consumption and high space requirements.
In this connection, many attempts have been made to develop a more effective process for separating low molecular weight compounds from polyamide 6 after its synthesis, in particular by heating the polymer melt in vacuo. It was possible tc achieve a residual content of low molecular weight compounds of 2 to 3% under laboratory conditions and of 3 to 5% under production conditions. ("Synthetische Polymer-Fasern" [Synthetic Polymer Fibers], T. Klare et al. 1966, Verlag "Mir", P. 171-172). A content of low molecular weight compounds in the polymer, after separation of the monomers, of 1.6 to 2%, and in the freshly formed fibers of 2.0 to 2.5% was achieved only with a relatively long duration of the monomer elimination process (8 to 10 hours) in vacuo, and in combination with a high melt temperature (SU-inventor's certificate No. 2882224, published Dec. 3, 1970, 29B, 3/60).
The chief difficulty in removing low molecular weight compounds (LMWCs) from polycaproamide melt is that the LMWCs contain about 1.5% (relative to the polymer) of high-melting, cyclic dimers, which cannot be separated from the polymer melt by heating the latter in vacuo at temperatures of 240.degree. to 280.degree. C, since their melting point is above 340.degree. C. (CZ Chemie-Technik, volume 2, 1973, No. 3, P. 95-99).
Increasing the process temperature to 300.degree. C and above in order to separate these compounds has the disadvantage that it displaces the equilibrium in the polymer-monomer system in the direction of increasing the quantity of monomers.
Moreover, additional difficulties arise in carrying out this process, since the cyclic .epsilon.-caprolactam dimer is easily sublimed and its solubility in caprolactam is slight. It can therefore easily enter the condenser system of the evaporator, where it is deposited in such a way that blockages and consequent malfunctions can occur in the evaporation.
Various processes are known for separating low molecular weight compounds from the polyamide 6 melt by heating it in vacuo in various evaporators, such as thin-layer apparatuses or extruders which are fitted with degassing devices.
According to Federal German Patent 2,503,308 C08G, 69/46, published Mar. 10, 1983 "Verfahren zur Gewinnung von Polykaprolaktam-Schmelze mit geringem Gehalt an Monomeren" [Process for producing polycaprolactam melt with a low content of monomers](Applicant, Bayer, Federal Republic of Germany) the polymer melt, which has not attained the equilibrium state, having a monomer content of 12 to 30% and a relative viscosity of 2.2 to 2.3, is subjected to heating in vacuo, and monomer (caprolactam) and cyclic dimer are separated in an evaporator (preferably a thin-layer apparatus) in a ratio of greater than 150:1 with a residual pressure of 6 to 60 Torr (6 to 60 mm Hg or 7.98-79.80 mbar). In this case, small amounts of dimer are evaporated simultaneously with the large amount of .epsilon.-caprolactam and remain dissolved in the resulting condensate.
The disadvantage of this process is by the authors' own account, a low utilization of the reactor due to a high content of low molecular weight monomer in the polymer and a high energy expenditure, which is associated with the distillation of a large amount of monomer.
According to Federal German Patent application No. 2,821,686, C08G, 69/19, published Nov. 22, 1979 "Verfahren zur Gewinnung von Polyamiden mit niedrigem Gehalt an Monomeren und Dimeren" [Process for the production of polyamides with a low content of monomers and dimers], a process is proposed which represents the further development of the process according to patent No. 2,503,308 and eliminates some of its disadvantages. A polycaproamide melt, which is in equilibrium, having a content of 7-8% of monomers and dimers and a specific viscosity of 2.55-2.78, is subjected to heating in vacuo at elevated temperatures, preferably 250.degree.-290.degree. C, and at a high vacuum, 0.3-5 Torr (0.3-5 mm Hg or 0.4-6.55 mbar), preferably 0.3-2 Torr (0.3-2 mm Hg or 0.4-2.66 mbar).
The difference with this process lies in the condensation of the low molecular weight compounds using the caprolactam melt which is being fed into the polymerization, thus preventing the deposit of condensed high-melting dimer in the pipework.
It is seen from the examples listed in the patent, that the residual content of monomer and dimers is 0.3 to 0.45%. The residual content of other oligomers is, however, not given in the examples; these also belong, as mentioned above, to the components of the low molecular weight compounds which are difficult to separate and their content in the original polycaproamide melt is about 1.6% (CZ Chemie-Technik, volume 2, 1973, No. 3, P. 95-99). Therefore, the residual content of low molecular weight compounds in this case may well be a total of 2%.
The high vacuum of 0.4 to about 6 mbar used in this process makes the design of the equipment considerably more difficult due to its sealing and due to reaching such degrees of vacuum. Moreover, as seen from the examples listed, such a treatment of polycaproamide melt, both in a thin-layer apparatus, and in a twin-screw extruder, leads in any case to an increase in the relative viscosity from 2.65-2.78 to 2.99-3.27, which is not always expedient.
The experiments carried out by the present inventors for the purpose of degassing polycaproamide melt by heating in vacuo on a twin screw extruder with 3 degassing zones, supplied by Werner & Pfleiderer GmbH, Federal Republic of Germany, using a residual pressure of 5 Torr (6.5 mbar) and a processing temperature range from 240.degree. to 300.degree. C revealed no actual possibility of achieving the desired residual content of low molecular weight compounds. The minimum value achieved was 1.8%. An increase in the relative viscosity from 2.72 to 3.12-3.55 was observed simultaneously with the separation of the low molecular weight compounds.
In the industrial production of polycaproamide, the use is known of the process of heating the polymer melt in vacuo in monomer distillation apparatuses, which function as continuous thin-layer apparatuses at a residual pressure of 1.33-13.3 mbar and at a temperature of 260.degree. to 270.degree. C. In this case, a residual content of LMWCs of .gtoreq.3% is achieved within 6 to 8 hours, with a simultaneous increase in the relative polymer viscosity from 2.5 to 3.1-3.2. (Frischmann, K.E., Chruzin, N.A. "Herstellung der Kapronfaser" [Production of caproic fibers]M., Verlag "Chimija", 1976, P. 87-94). A disadvantage of this process is the concurrence of thermal degradation processes, which lead to gel formation in the polymer and to a worsening of polymer quality.
Processes are also known for separating low molecular weight compounds from the polyamide 6 melt using inert gas or steam (inventor's certificate No. 817032, published Mar. 30, 1981, C08G, 69/16; Federal German Patent Application No. 2,461,901, published July 8, 1976, C08G, 69/46; inventor's certificate No. 149,873, published May 9, 1966, BOld; Federal German Patent Application No. 2,340,261, published Mar. 6, 1975).
According to the process of inventor's certificate No. 817032, the separation of the low molecular weight compounds from the polycaproamide melt is carried out in an apparatus with a mixing device in the presence of superheated steam, the amount of the latter relative to the amount of low molecular weight compounds contained in the polymer before their distillation being 0.1:1 to 1:1. This process allows the production of a polymer with given molecular weights and with a reduced content of products arising from the undesired side reactions. According to this process, a residual content of low molecular weight compounds in the polymer of 3 to 6% is achieved, with simultaneous increase in the relative viscosity from 2.24 to 3.1-3.4.
According to the process of Federal German Patent Application No. 2,461,901, steam or nitrogen are used in a thin-layer evaporator and are fed through a perforated surface and intensively mixed with the polyamide film. The process affords a residual content of low molecular weight compounds of a minimum of 2%. Inventor's certificate No. 149,873 relates to the details of the design of the thin-layer apparatus, which permit the use of steam.
The technical solution (prototype) which comes next is a process for the separation of the oligomer components (low molecular weight compounds) from polyamides (Federal German Patent Application No. 2,340,261, C08G, 69/46, published Mar. 6, 1975). According to this process the separation of the low molecular weight compounds from the polyamide 6 melt is carried out in a continuous screw compounder (preferably of the ZSK type with intermeshing, self-cleaning screws), in which according to the description, saturated steam is introduced into the polymer melt at the melt temperature and is mixed with the melt. By virtue of the appropriate screw geometry, the pressure in the melt is increased and condensation of the introduced steam is brought about with the evolution of a certain amount of heat. The pressure in the system is then drastically reduced (but not below atmospheric pressure) by the appropriate construction of the screw elements, and the heat is led out of the system. The water evaporates together with the low molecular weight compounds and is led out of the extruder. The process can be carried out in steps in successively located zones.
The monomer components of the low molecular weight compounds are previously separated from the polymer melt, for example in two degassing zones, at a residual pressure of 1 to 10 Torr (1.33 to 13.33 mbar).
After separation of the oligomer components from the polyamide 6 melt, the latter is further subjected in the final step to vacuum degassing in order to separate remaining traces of monomers and water.
This process comes closest in technical character to the present invention.
Disadvantages of the prototype are:
1. The saturated steam used is to have the temperature of the polyamide 6 melt, i.e. 240.degree. to 280.degree. C, which corresponds to a steam pressure of 33.3-64.2 bar. In this case, it is necessary that the pressure of the polymer melt at the point of introduction of the saturated steam have a value somewhat lower than the latter values, but not much lower, since otherwise the steam can escape from the extruder in the direction opposite to the movement of melt, i.e. in the direction of lower resistance.
In the case of polyamide 6 melt, the maximum pressure which can be achieved in the steam incorporation zone of a twin screw compounder is about 20bar.
Ensuring a polymer melt pressure in the extruder of 33.3-64.2 bar is linked with the provision of screws with relatively high sealing properties, which can lead to local overheating due to high shear stresses. Moreover, the necessity arises here of extending the total length of the extruder with simultaneous loss of productivity.
2. The complete condensation of the saturated steam directly in the polymer melt, which is provided as a particular feature of the process, leads, as the author of the patent application correctly observes, to the evolution of a considerable amount of heat, namely 1593-1763 kJ/kg. Since in this case ideal conditions arise for heat transfer, heat being liberated in such amounts can increase the temperature of the polyamide melt by dozens of degrees in a short time.
For its part, this should lead to the displacement of the equilibrium in the polymer-monomer system in the direction of increasing the content of caprolactam, which must also be separated, in order to obtain the desired quality of the finished product. Consequently, additional efforts are necessary in order to separate and regenerate the said caprolactam.
3. The overheating of the polycaproamide melt in the presence of condensed water creates favorable conditions for the processes of thermal decomposition and hydrolysis with the development of undesired side reactions and the formation of branched products which always leads to a reduction of the molecular weight of the polymer and to gel formation.
4. The final stage of treating the polymer melt in vacuo, provided in the process, leads to dehydration of the polymer melt, which for its part according to existing data (for example Reining G., Golke U., "Uber das Polykondensationsstadium bei der Gewinnung von Polykaproamid" [Concerning the polycondensation stage in the production of polycaproamide], Hochmolekulare Verbindungen, 1973, Vol. (A) 15, No. 2, P. 372-379 ) causes the development of side reactions with the formation of branched structures and gel formation. Moreover, during the transport and processing of the dehydrated melt an uncontrollable increase occurs in the polymer viscosity. Therefore, the processing of the demonomerized melt to products which are subjected to orientation hardening is made more difficult and sometimes impossible.
5. In order to carry out the process according to the description a twin screw compounder with at least 4 degassing zones is necessary (two in order to remove monomer, at least one in order to remove the oligomers, and one in order to remove the remaining traces of monomer and water) . Equipment of this type is quite complex and very expensive.